Editing Charge invariance (section)

{{Short description|Principle in particle physics}}
{{Unreferenced stub|auto=yes|date=December 2009}}
{{main|Covariant formulation of classical electromagnetism#Electric charge}}

'''Charge invariance''' refers to the fixed value of the [[electric charge]] of a particle regardless of its motion. Like [[Invariant mass|mass]], total [[Relativistic angular momentum#Spin in special relativity|spin]] and [[magnetic moment]], particle's charge quantum number remains unchanged between two [[Frame of reference|reference frames]] in relative motion. For example, an [[electron]] has a specific charge ''e'', total spin <math>\tfrac{\hbar}{2}</math>, and invariant mass ''m''<sub>e</sub>.  Accelerate that electron, and the charge, spin and mass assigned to it in all physical laws in the frame at rest and the moving frame remain the same – ''e'', <math>\tfrac{\hbar}{2}</math>, ''m''<sub>e</sub>. In contrast, the particle's total relativistic [[energy]] or [[de Broglie wavelength]] change values between the reference frames.  

The origin of charge invariance, and all relativistic invariants, is presently unclear.  There may be some hints proposed by [[string theory|string]]/[[M-theory]].  It is possible the concept of charge invariance may provide a key to unlocking the mystery of [[theory of everything|unification]] in physics – the single theory of [[gravity]], [[electromagnetism]], the [[strong nuclear force|strong]], and [[weak nuclear force]]s.

The property of charge invariance is embedded in the charge density – current density [[four-vector#Four-current|four-vector]] <math>j^\mu = \left(c\rho, \vec{j}\right)</math>, whose vanishing divergence <math>\partial_\mu j^\mu = 0</math> then signifies [[charge conservation]].